JP5354582B2 - Terahertz wave generator - Google Patents

Description

  The present invention relates to an apparatus for generating a terahertz wave having a frequency between microwaves (radio waves) and infrared rays (light waves).

  Terahertz waves having a frequency between radio waves and light waves are very effective as light sources for internal defect inspection, reagent inspection, and enantiomeric content analysis of ceramics. Development is actively underway. FIG. 7 shows the relationship between the frequency of the light source published so far and the output. As shown in FIG. 7, in the frequency band below 0.1 THz and the frequency band higher than several tens THz, a plurality of types of light sources are published. On the other hand, in the frequency band in the meantime, there is a frequency band for which an effective light source has not yet been disclosed. A terahertz wave light source capable of high output in a wide frequency band is desired.

  Patent Document 1 discloses one technique for generating a terahertz wave using a difference frequency generation method. In this technique, laser beams having two different frequencies are incident on a terahertz wave generating crystal, and a terahertz wave corresponding to the frequency difference is generated. When the laser light is incident on the crystal for generating the terahertz wave, the laser light and the terahertz wave are incident so that the phase conditions are matched. Generally, the refractive index in the frequency band of terahertz waves is larger than the refractive index in the frequency band of laser light. Therefore, in the technique of Patent Document 1, when the phase of the laser light and the terahertz wave is matched, the two laser lights are used for generating the terahertz wave with a finite angle difference between the two laser lights having different frequencies. Incident on the crystal. It has been confirmed that a terahertz wave having a frequency of 0.6 to 2.6 THz is generated by using the technique of Patent Document 1.

JP 2006-215222 A

The technique of Patent Document 1 is a very effective technique for developing a terahertz light source.
However, in the technique of Patent Document 1, as the frequency of the terahertz wave decreases, the angle difference between the two laser beams incident on the terahertz wave generating crystal decreases. When the angle difference is small, it is difficult to adjust the angle difference correctly, and the energy conversion efficiency when generating the terahertz wave from the laser light is deteriorated. In FIG. 6, the relationship between the frequency of the terahertz wave generated using the technique of Patent Document 1 and the energy conversion efficiency is indicated by a dotted line. As shown in FIG. 6, in the region where the frequency is less than 1 THz, an event has been observed in which the energy conversion efficiency of the terahertz wave deteriorates as the frequency decreases. When the terahertz wave travels in the terahertz wave generating crystal, the terahertz wave is absorbed by the terahertz wave generating crystal in accordance with the distance traveled in the terahertz wave generating crystal. In general, absorption of terahertz waves into a crystal for generating terahertz waves becomes significant at high frequencies. Therefore, as shown by a dotted line in FIG. 6, in the region where the frequency is 2 THz or more, an event in which the energy conversion efficiency of the terahertz wave deteriorates as the frequency increases is observed. When the energy conversion efficiency of terahertz waves deteriorates, it is difficult to output high-power terahertz waves. In order to generate a high-output terahertz wave, improvement in energy conversion efficiency is desired.
In view of the above problems, an object of the present invention is to provide a technique for improving the energy conversion efficiency of a terahertz wave in a wide frequency band.

The present invention generates a terahertz wave using a Cherenkov phase matching method, and further devise the thickness of a nonlinear optical crystal that is a terahertz wave generating crystal, thereby converting energy when generating a terahertz wave from a laser beam. Succeeded in improving efficiency.
The present invention is embodied in a terahertz wave generator. In this terahertz wave generator, two types of laser beams having different frequencies are incident on a nonlinear optical crystal having a second-order nonlinearity, and a terahertz wave having a difference frequency between the two types of laser beams is emitted by a Cherenkov phase matching method. To do.
In the terahertz wave generator of the present invention, two types of laser light are incident collinearly on the incident surface of the nonlinear optical crystal. The “incident surface” means a surface on which laser light is incident among the surfaces of the nonlinear optical crystal. In addition, the thickness of the incident surface is set to be smaller than a half wavelength in the terahertz wave nonlinear optical crystal. A buffer layer is formed on at least one of the radiation surfaces of the nonlinear crystal. The “radiation surface” means a surface that emits terahertz waves among the surfaces of the nonlinear optical crystal. The refractive index of the buffer layer in the visible to near-infrared frequency band is set smaller than the refractive index of the nonlinear optical crystal in the visible to near-infrared frequency band.

  When laser light is incident on the nonlinear optical crystal, nonlinear polarization occurs in the crystal. In a nonlinear optical crystal having a second-order nonlinearity, when two laser beams having different frequencies are incident, nonlinear polarization having a period corresponding to the difference frequency between the two laser beams is generated. In the nonlinear optical crystal, the energy state is excited by the incidence of laser light, and an energy wave is emitted when returning to the original energy state. When the nonlinear optical crystal is nonlinearly polarized, an energy wave corresponding to the polarization frequency is emitted. When the nonlinear optical crystal is polarized with a terahertz wave frequency, a terahertz wave is emitted from the nonlinear optical crystal. However, in general, the refractive index of terahertz waves is larger than the refractive index of laser light, and the propagation speed of light in nonlinear optical crystals exceeds the propagation speed of terahertz waves in nonlinear optical crystals. Therefore, terahertz waves are emitted from the nonlinear optical crystal by Cherenkov. The terahertz wave is emitted from the nonlinear optical crystal to a radiation angle that satisfies the Cherenkov-type phase matching condition. In the present invention, terahertz waves are radiated by the Cherenkov phase matching method.

  In the present invention, terahertz waves are radiated by the Cherenkov type phase matching method. In the Cherenkov phase matching method, a terahertz wave is radiated based on nonlinear polarization generated in a nonlinear optical crystal. In addition, when generating nonlinear polarization, it is not necessary to make an angle difference between the two laser beams. Therefore, the first laser beam and the second laser beam can be incident on the nonlinear optical crystal collinearly. Compared with the technique of Patent Document 1 in which the first laser beam and the second laser beam are adjusted to a predetermined angle difference, the angular relationship between the first laser beam and the second laser beam can be adjusted accurately, and the terahertz wave The energy conversion efficiency in the low frequency region can be improved.

In the terahertz wave generator of the present invention, the thickness of the incident surface is formed to be smaller than a half wavelength in the terahertz wave nonlinear optical crystal.
The beam diameter of the laser light incident on the nonlinear optical crystal has a finite irradiation area, and an infinite number of minute laser lights are incident on the inside. The minute laser light nonlinearly polarizes the nonlinear optical crystal at the incident site and emits a minute terahertz wave from each site. As a result, a terahertz wave that is a collection of micro terahertz waves (micro terahertz wave group) is emitted from the nonlinear optical crystal. The phase of the minute terahertz wave radiated from each position of the nonlinear optical crystal at the radiation surface of the nonlinear optical crystal is determined based on the distance in the thickness direction of the incident surface at that position. If a terahertz wave contains a micro terahertz wave whose phase on the radiation surface of the nonlinear optical crystal is opposite, the micro terahertz wave cancels each other and is weakened, and the terahertz wave emitted from the nonlinear optical crystal is emitted. The energy conversion efficiency of will deteriorate.
In the present invention, by setting as described above, it is possible to suppress a minute terahertz wave having an antiphase in the minute terahertz wave group. Thereby, it is suppressed that the terahertz wave is weakened, and the energy conversion efficiency in the high frequency region of the terahertz wave can be improved.

By setting the thickness of the incident surface of the nonlinear optical crystal to be small as described above, it is possible to obtain an effect of suppressing the amount of terahertz waves absorbed in the nonlinear optical crystal.
When the terahertz wave travels in the nonlinear optical crystal, the terahertz wave is absorbed by the nonlinear optical crystal in accordance with the distance traveled in the nonlinear optical crystal. By setting the nonlinear optical crystal as described above, the distance that the terahertz wave travels in the nonlinear optical crystal can be shortened, and the amount of the terahertz wave absorbed by the nonlinear optical crystal can be suppressed. Thereby, the energy conversion efficiency of the terahertz wave can be improved.

In the terahertz wave generator of the present invention, the buffer layer is formed on at least one of the radiation surfaces, and the refractive index in the frequency band from the visible light to the near infrared of the buffer layer is the visible of the nonlinear optical crystal. It is set smaller than the refractive index in the frequency band from light to near infrared.
According to the present invention, the first laser light and the second laser light incident on the nonlinear optical crystal are likely to be totally reflected at the boundary between the nonlinear optical crystal and the buffer layer. That is, leakage of the first laser light and the second laser light from the nonlinear optical crystal is suppressed, and the nonlinear optical crystal can function as a waveguide through which the first laser light and the second laser light pass. As a result, the attenuation of the first laser light and the second laser light incident on the nonlinear optical crystal is suppressed, and the energy conversion efficiency of the terahertz wave can be improved.

  It is preferable that a prism is provided on the surface of the buffer layer. The refractive index of the nonlinear optical crystal in the visible to near-infrared frequency band is n1 (OPT), the refractive index of the nonlinear optical crystal in the terahertz wave frequency band is n1 (THz), and the prism in the terahertz wave frequency band. When the refractive index is n2 (THz), the refractive index n1 (OPT), the refractive index n1 (THz), and the refractive index n2 (THz) are configured to satisfy the relationship of Equation 1.

  FIG. 5 shows a state in which the terahertz wave L3 is radiated from the nonlinear optical crystal 26 to the prism 30. In FIG. 5, the buffer layer is omitted. The radiation angle θ of the terahertz wave with respect to the traveling direction of the first laser beam L1 and the second laser beam L2 is determined by the phase condition. That is, the wavelength of the first laser light L1 is λ1, the wavelength of the second laser light L2 is λ2, the wavelength of the terahertz wave in vacuum is λ (THz), and the frequency band from the visible light to the near infrared of the nonlinear optical crystal Assuming that the refractive index of the first laser light L1 at n is n11 (OPT) and the refractive index of the second laser light L2 in the frequency band from the visible light to the near infrared of the nonlinear optical crystal is n12 (OPT), It can be expressed as shown in Equation 3 by approximation.

  The critical angle α0 when the terahertz wave is radiated from the nonlinear optical crystal 26 to the prism 30 is expressed by Equation 4 using the refractive index n2 (THz) in the frequency band of the terahertz wave of the prism. The terahertz wave is radiated from the nonlinear optical crystal 26 to the prism 30 when the incident angle α radiated from the nonlinear optical crystal 26 to the prism 30 satisfies the relationship of Equation 5. Further, the incident angle α and the radiation angle θ have the relationship of Equation 6. By using Equations 3 to 6, the relationship of Equation 1 can be obtained.

In the present invention, the terahertz wave radiated from the nonlinear optical crystal can be incident on the prism by satisfying the relationship of Equation 1.
According to the present invention, a terahertz wave can be radiated to a desired position using a prism. Further, by selecting a material having a small amount of terahertz wave absorption as the material of the prism, it is possible to suppress deterioration of the energy conversion efficiency of the terahertz wave.

  According to the present invention, the energy conversion efficiency of terahertz waves can be improved in a wide frequency band. Thereby, the output of the terahertz light source can be improved.

It is the figure which showed typically the terahertz wave generator 10 and the terahertz wave measuring device 60. FIG. 2 is a diagram schematically showing the configuration of a waveguide device 20. FIG. It is a figure explaining a mode that a terahertz wave is emitted. It is a figure explaining a mode that a terahertz wave is emitted. It is a figure explaining a mode that a terahertz wave is emitted. The relationship between the frequency of the terahertz wave radiated | emitted from the terahertz wave generator 10 of a present Example and energy conversion efficiency is shown. The relationship between the frequency and output of the terahertz light source that has been published so far is shown.

The main features of the embodiment described below will be summarized.
(Feature 1) The first laser beam and the second laser beam have frequencies in the frequency band from visible light to near infrared.
(Feature 2) The nonlinear optical crystal is formed in layers. The side surface of the nonlinear optical crystal corresponds to the incident surface, and at least one of the upper surface and the bottom surface of the nonlinear optical crystal corresponds to the radiation surface.
(Characteristic 3) The thickness of the buffer layer is larger than the seepage length of the first laser beam and the second laser beam.
(Feature 4) The thickness of the buffer layer is smaller than the wavelength in the terahertz wave buffer layer.
(Feature 5) The buffer layer is formed of a material having a small absorption coefficient for terahertz waves.

  Embodiments of the present invention will be described with reference to the drawings. FIG. 1 schematically shows a terahertz wave generation device 10 of the present invention and a terahertz wave measurement device 60 that measures a terahertz wave L3 emitted from the terahertz wave generation device 10. The terahertz wave generator 10 includes a YAG laser oscillator 12, a KTP optical parametric oscillator (KTP-OPO) 14, a lens 16, and a waveguide device 20. The terahertz wave measuring device 60 includes a bolometer 62, an A / D converter 64, and a PC 66. Between the terahertz wave generation device 10 and the terahertz wave measurement device 60, a lens 68 for condensing the terahertz wave L3 is disposed.

  The terahertz wave generator 10 of the present invention will be described. The YAG laser oscillator 12 is a laser oscillator using a YAG crystal to which Nd ions are added, and makes a laser beam L0 having a wavelength of 532 nm incident on the KTP-OPO14. The KTP-OPO 14 is an optical parametric oscillator using a KTP crystal, and generates a laser beam group (wavelength: 1300 nm to 1550 nm) having a wavelength approximately three times from the laser beam incident from the YAG laser oscillator 12. Yes. The KTP-OPO 14 selects a first laser beam having a frequency ω1 (sometimes referred to as pump light) and a second laser beam having a frequency ω2 (sometimes referred to as idler light) from the generated laser light group. These two laser beams are incident on the waveguide device 20 through a collinear line. In the KTP-OPO 14, the difference frequency ω3 (= ω1−ω2) between the first laser beam L1 and the second laser beam L2 is set to be a desired terahertz wave frequency.

FIG. 2 schematically shows the configuration of the waveguide device 20. The waveguide device 20 is configured by laminating a crystal stage 22, an adhesive layer 24, a nonlinear optical crystal 26, and a buffer layer 28 prism 30 in that order.
The crystal stage 22 is a stage on which the nonlinear optical crystal 26 is arranged, and a layered nonlinear optical crystal 26 is laminated on the upper surface of the crystal stage 22 with an adhesive layer 24 interposed therebetween. The nonlinear optical crystal 26 is a crystal having second-order nonlinearity, for example, a crystal such as LiNbO 3, LiTaO 3, DAST, ZnTe, GaAs, ZnSe, GaSe, and GaP. The thickness of the nonlinear optical crystal 26 is set to be smaller than half of the wavelength of the terahertz wave nonlinear optical crystal 26 having the frequency ω3. Here, the wavelength of the terahertz wave having the frequency ω3 in the nonlinear optical crystal 26 is determined by using the wavelength λ (THz) in the vacuum of the terahertz wave L3 and the refractive index n1 (THz) in the frequency band of the terahertz wave of the nonlinear optical crystal 26. , Λ (THz) / n1 (THz). As will be described later, when the first laser beam L1 and the second laser beam L2 are incident on the side surface 26a, the nonlinear optical crystal 26 emits a terahertz wave having a frequency ω3 from the upper surface 26b.
A buffer layer 28 is stacked on the upper surface 26 b of the nonlinear optical crystal 26 that emits terahertz waves, and a prism 30 is stacked on the upper surface of the buffer layer 28.

The thickness d of the buffer layer 28 is formed to be thicker than the seepage length ξ of the first laser light L1 and the second laser light L2. The buffer layer 28 is a substance whose refractive index n3 (OPT) in the frequency band from visible light to near infrared is lower than the refractive index n1 (OPT) in the frequency band from visible light to near infrared of the nonlinear optical crystal 26. It is formed with. In the waveguide device 20, the first laser light L 1 and the second laser light L 2 incident on the nonlinear optical crystal 26 are suppressed from oozing out to the buffer layer 28, and the nonlinear optical crystal 26 functions as a waveguide. be able to.
On the other hand, the thickness d of the buffer layer 28 is set thinner than the wavelength in the terahertz wave buffer layer. The buffer layer 28 is made of a material having a small absorption coefficient for terahertz waves. Therefore, when the terahertz wave is propagated from the nonlinear optical crystal 26 to the prism 30 via the buffer layer 28, the terahertz wave is suppressed from being attenuated in the buffer layer 28. Therefore, in the following description, the buffer layer 28 may be omitted when considering the propagation of the terahertz wave from the nonlinear optical crystal 26 to the prism 30.

  Next, the terahertz wave measuring device 60 will be described. The bolometer 62 is a Si bolometer and includes a measurement port 62a. The bolometer 62 measures the energy of the terahertz wave incident through the measurement port 62a in association with the frequency of the incident terahertz wave. The measurement port 62 a is disposed to face the waveguide device 20 via the lens 68. Specifically, it is directed in the direction in which the terahertz wave is emitted from the waveguide device 20. The bolometer 62 transmits an analog value, which is a measured energy value, to the PC 66. The energy value transmitted from the bolometer 62 is digitized by the A / D converter 64 and transmitted to the PC 66. The PC 66 calculates the relationship between the frequency and energy value of the terahertz wave emitted from the terahertz wave generation device 10 using the transmitted energy value.

The manner in which terahertz waves are radiated from the waveguide device 20 will be described with reference to the drawings. As described above, when the terahertz wave is propagated from the nonlinear optical crystal 26 to the prism 30, the buffer layer 28 can be omitted, and the buffer layer 28 is omitted in the drawings used for the following description. Show.
As shown in FIG. 3, when the first laser beam L1 and the second laser beam L2 are incident on the side surface 26a of the nonlinear optical crystal 26, nonlinear polarization occurs in the nonlinear optical crystal 26 along the traveling direction of the laser beams L1 and L2. Occurs. The wave shown in the lower part of FIG. 3 schematically shows the state of polarization inside the nonlinear optical crystal 26. When the nonlinear optical crystal 26 has second-order nonlinearity, nonlinear polarization having a period 2Lc corresponding to the difference frequency ω3 between the laser beams L1 and L2 occurs. Here, the period 2Lc indicates the refractive index n11 of the first laser beam L1 in the frequency band from the wavelength λ1 of the first laser beam L1, the wavelength λ2 of the second laser beam L2, and the visible light to the near-infrared wavelength of the nonlinear optical crystal 26. OPT) and the refractive index n12 (OPT) of the first laser light L1 in the frequency band from the visible light to the near-infrared of the nonlinear optical crystal 26 are expressed as shown in Equation 7.

In the nonlinear optical crystal 26, the energy state is excited by the incidence of the first laser beam L1 and the second laser beam L2. The excited nonlinear optical crystal 26 emits an energy wave when returning to the original state. When the nonlinear optical crystal 26 is polarized according to the difference frequency ω3, a terahertz wave L3 having the difference frequency ω3 is emitted. At this time, the propagation speed of visible light in the nonlinear optical crystal 26 exceeds the propagation speed of the terahertz wave L3 in the nonlinear optical crystal 26, and the terahertz wave L3 is emitted from the nonlinear optical crystal 26 by Cherenkov.
In the nonlinear optical crystal 26, the terahertz wave L3 is radiated in a spherical shape from the point X1 on the side surface 26a on which the first laser beam L1 and the second laser beam L2 are incident, and then the terahertz wave L3 is spheroidized from the point X2. Radiated. The terahertz wave L3 radiated from the point X2 has the same phase as the terahertz wave L3 radiated from the point X1, and is delayed by one cycle. Similarly, from the point X3, the phase is the same as that of the terahertz wave L3 radiated from the point X2, and the terahertz wave L3 delayed by one cycle is radiated in a spherical shape. From the point X4, the phase is the same as that of the terahertz wave L3 radiated from the point X3, and the terahertz wave L3 delayed by one cycle is radiated in a spherical shape. From the point X5, the phase is the same as that of the terahertz wave L3 emitted from the point X4, and the terahertz wave L3 delayed by one cycle is emitted in a spherical shape.

  The terahertz wave L3 simultaneously radiated from the points X1 to X5 propagates in a direction perpendicular to the common tangent line of each wavefront. The radiation angle θ (sometimes called the Cherenkov angle) of the terahertz wave L3 with respect to the traveling direction of the laser light is expressed as in Expression 8 using the wavelength λ (THz), the refractive index n1 (THz), and the period 2Lc. The The relationship of Equation 2 can be obtained from Equation 7 and Equation 8, and the relationship of Equation 3 can be obtained by further approximation. The upper part of FIG. 3 schematically shows a state in which the terahertz wave L3 is radiated with a radiation angle θ inside the nonlinear optical crystal 26.

  The first laser beam L1 and the second laser beam L2 incident on the nonlinear optical crystal 26 have a finite irradiation area. Therefore, as shown in FIG. 4, the first laser beam L1 and the second laser beam L2 are simultaneously incident at different depth positions P1 and P2 of the nonlinear optical crystal 26, and terahertz waves L3 are emitted from the positions P1 and P2, respectively. The In general, the phases of the terahertz wave L3 emitted from the position P1 and the terahertz wave emitted from the position P2 on the upper surface 26b of the nonlinear optical crystal 26 are different. When the phase of the terahertz wave L31 radiated from the position P1 and the phase of the terahertz wave L32 radiated from the position P2 on the upper surface 26b of the nonlinear optical crystal 26 are reversed, the terahertz waves L3 radiated from the respective positions cancel each other. The terahertz wave L3 output from the nonlinear optical crystal 26 is weakened.

  In the nonlinear optical crystal 26 of the present embodiment, the thickness D is set to be thinner than half of the wavelength λ (THz) / n1 (THz) in the nonlinear optical crystal 26 of the terahertz wave L3 having the frequency ω3. Therefore, the phase difference between the terahertz wave L3 radiated from the upper surface 26b of the nonlinear optical crystal 26 having the most different phase and the terahertz wave L3 radiated from the lower surface 26c of the nonlinear optical crystal 26 on the upper surface 26b of the nonlinear optical crystal 26 is expressed by “π "Can be kept smaller. This suppresses the terahertz waves L3 radiated from the different depth positions of the nonlinear optical crystal 26 from weakening each other.

  In the nonlinear optical crystal 26 of the present embodiment, the distance that the terahertz wave L3 travels in the nonlinear optical crystal 26 can be shortened by setting the thickness D of the nonlinear optical crystal 26 thin as described above. . Thereby, the amount of the terahertz wave L3 absorbed by the nonlinear optical crystal 26 can be reduced.

  The terahertz wave L3 radiated from each point of the nonlinear optical crystal 26 is radiated from the upper surface of the nonlinear optical crystal 26 to the prism 30 through the buffer layer 28. As described above, the refractive index n1 (THz) in the frequency band of the terahertz wave L3 of the prism 30 is set to be smaller than the refractive index n2 (THz) of the nonlinear optical crystal 26 in the frequency band of the terahertz wave L3. Therefore, as shown in FIG. 5, the terahertz wave L <b> 3 radiated from the nonlinear optical crystal 26 is refracted when entering the prism 30.

  The incident angle α of the terahertz wave L3 to the prism 30 is expressed as in Expression 6 using the radiation angle θ. In addition, the critical angle α0 when the terahertz wave L3 is incident on the prism 30 from the nonlinear optical crystal 26 is expressed as Equation 4. Therefore, when the incident angle α satisfies the relationship of Equation 5, the terahertz wave L3 is radiated from the nonlinear optical crystal 26 to the prism 30. From Equation 3 to Equation 6, the relationship of Equation 1 is obtained. In the waveguide device 20 of the present embodiment, the terahertz wave L3 generated in the nonlinear optical crystal 26 is radiated to the prism 30 when the relationship of Equation 1 is satisfied. The terahertz wave radiated to the prism 30 is radiated from the upper surface 30a of the prism 30 to the atmosphere.

FIG. 6 shows the relationship between the frequency of the terahertz wave L3 radiated from the terahertz wave generator 10 of the present embodiment measured using the terahertz wave measuring device 60 and the energy conversion efficiency. In FIG. 6, the relationship between the frequency of the terahertz wave radiated using the terahertz wave generating device of Patent Document 1 and the energy conversion efficiency is indicated by a dotted line for comparison.
In the terahertz wave generator 10 of the present embodiment, the first laser light L1 and the second laser light L2 are incident on the nonlinear optical crystal 26 of the waveguide device 20 in a collinear manner. There is no need to provide an angle difference between the first laser beam L1 and the second laser beam L2 as in the technique of Patent Document 1. Therefore, the first laser light L1 and the second laser light L2 can be accurately adjusted, and the energy conversion efficiency of the terahertz wave L3 can be improved.

  In the terahertz wave generator 10 of the present embodiment, the thickness D of the nonlinear optical crystal 26 is more than half of the wavelength λ (THz) / n1 (THz) in the nonlinear optical crystal 26 of the terahertz wave L3 having the frequency ω3. It is set thinly. Thereby, the terahertz wave L3 radiated from the nonlinear optical crystal 26 is suppressed from being attenuated in the nonlinear optical crystal 26, and the energy conversion efficiency of the terahertz wave L3 can be improved.

Specific examples of the present invention have been described in detail above, but these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and changes of the specific examples illustrated above.
For example, the prism 30 of the present embodiment is preferably formed so that the upper surface 30a thereof is perpendicular to the radiation direction of the terahertz wave L3. Since the upper surface 30a of the prism 30 is formed as described above, energy loss is suppressed when the terahertz wave L3 is emitted from the prism 30 into the atmosphere, and the extraction efficiency of the terahertz wave L3 is increased. can do.

  The same applies to the boundary between the nonlinear optical crystal 26 and the prism 30. The nonlinear optical crystal 26 is preferably formed so that its upper surface 26b is perpendicular to the radiation direction of the terahertz wave L3. Thus, when the terahertz wave L3 is radiated from the nonlinear optical crystal 26 to the prism 30, energy loss is suppressed, and the energy conversion efficiency of the terahertz wave L3 can be increased.

  In the waveguide device 20 of the present embodiment, the terahertz wave L3 is radiated from the upper surface 26b of the nonlinear optical crystal 26, but the terahertz wave L3 may be radiated from the lower surface 26c of the nonlinear optical crystal 26. Further, the terahertz wave L3 may be simultaneously emitted from both the upper surface 26b and the lower surface 26c of the nonlinear optical crystal 26. By radiating the terahertz wave L3 from both sides, the energy conversion efficiency of the terahertz wave L3 can be improved.

  In addition, the technical elements described in the present specification or drawings exhibit technical usefulness alone or in various combinations, and are not limited to the combinations described in the claims at the time of filing. In addition, the technology illustrated in the present specification or the drawings achieves a plurality of objects at the same time, and has technical utility by achieving one of the objects.

DESCRIPTION OF SYMBOLS 10 Terahertz wave generator 12 YAG laser oscillator 14 KTP optical parametric oscillator 16 Lens 20 Waveguide device 22 Crystal stage 24 Adhesive layer 26 Nonlinear optical crystal 28 Buffer layer 30 Prism 60 Terahertz wave measuring device 62 Bolometer 64 A / D converter 68 Lens L0 Laser light L1 First laser light (pump light)
L2 Second laser beam (idler beam)
L3 Terahertz wave α Incident angle α0 Critical angle θ Radiation angle (Cherenkov angle)
ω1 Frequency of the first laser light ω2 Frequency of the second laser light ω3 Frequency of the terahertz wave

Claims (2)

A terahertz wave generator that emits terahertz waves having a difference frequency between the two types of laser light by using a Cherenkov phase matching method by entering two types of laser beams having different frequencies into a nonlinear optical crystal having second-order nonlinearity. There,
The two types of laser light are incident collinearly on an incident surface on which the laser light of the nonlinear optical crystal is incident,
The thickness of the incident surface is formed to be smaller than a half wavelength in the nonlinear optical crystal of the terahertz wave,
On at least one surface of the radiating surface for radiating the terahertz wave of the nonlinear optical crystal, the refractive index in the near infrared frequency band from visible light, refraction in the near infrared frequency band from visible light of the nonlinear optical crystal The buffer layer set smaller than the rate is formed ,
A prism is provided on the surface of the buffer layer,
Refractive index n1 (OPT) in the frequency band from visible light to near infrared of the nonlinear optical crystal, Refractive index n1 (THz) in the frequency band of the terahertz wave of the nonlinear optical crystal, and Frequency band of the terahertz wave of the prism The refractive index n2 (THz) at
arccos (n1 (OPT) / n1 (THz))> π / 2-arcsin (n2 (THz) / n1 (THz))
Is configured to satisfy
A terahertz wave generating device , wherein a radiation surface of the prism for emitting terahertz waves is formed to be perpendicular to a radiation direction of the terahertz waves traveling in the prism . 2. The terahertz wave generating device according to claim 1, wherein the buffer layer is formed on two radiation surfaces of the nonlinear optical crystal that emit terahertz waves. 3.

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